1. Field of the Invention
The present invention relates to a water-based ink set for ink-jet recording suitable for reducing graininess in a low density printed part and for extending a color reproduction range. The present invention also relates to an ink-jet recording method using this water-based ink set for ink-jet recording.
2. Description of the Related Art
When a color image is expressed by an ink-jet recording method, a three-color ink set has been generally used, composed of a yellow ink (Y), a magenta ink (M) and a cyan ink (C). Moreover, a four-color ink set has also been used in which a black ink (K) is further added to the standard three-color ink set.
Generally, in an ink-jet recording method, the gradation of an image is controlled by adjusting the density of dots formed by ejecting ink onto a recording material. However, when the gradation is controlled by such a method, the dot density decreases in a low density printed part. Relative to this, the individual dots become easily discernable to the eye, thereby causing the image to be grainy.
In view of the above, a method has been proposed in which two or more inks are employed as a cyan ink (Japanese Patent Application Laid-Open No. Hei 1-95093). In this method, these inks have different dye concentrations and are composed of different kinds of dyes. In addition, a dye which is excellent in vividness but inferior in light fastness is employed as a dark color ink, and a dye which is excellent in light fastness but inferior in vividness is employed as a light color ink. Moreover, a similar method has been proposed for a magenta ink (Japanese Patent Application Laid-Open No. Hei 2-127482). According to these methods, graininess can be improved, but a color reproduction range cannot be extended.
Generally, in a three-color ink set composed of yellow, magenta and cyan inks, and also in a four-color ink set in which a black ink is added to the three-color ink set, blue color is expressed by use of the cyan ink and the magenta ink, and green color is expressed by use of the cyan ink and the yellow ink. When these two inks are used to express blue color or green color as above, sharp printing quality and good color developing properties are difficult to achieve due to a landing error of superposition.
The present invention has been made to solve the abovementioned problems. Objects of the present invention are, in an ink-jet recording method:
(i) to reduce graininess in a low density printed part containing cyan color without reducing a color reproduction range in a blue direction and/or a green direction, and a cyan direction in a high density printed part, and with minimizing the number of inks composing an ink set; and
(ii) to extend the color reproduction range in the blue direction and/or the green direction to improve vividness.
The present inventors have conducted extensive studies on a water-based ink set for ink-jet recording. The studies are based on the hypothesis that the lightness (L*), the hue angle (h) and the chroma (C*) of the inks constituting the ink set are closely related to reducing graininess in a low density printed part of a color image and to extending the color reproduction range. Here, the lightness (L*), the hue angle (h) and the chroma (C*) are based on L*a*b* colorimetric system. Consequently, the present inventors have found that, in a water-based ink set for ink-jet recording having a cyan ink, the abovementioned objects (i) and (ii) can be achieved by employing a light cyan ink as the cyan ink and also employing a blue ink and/or a green ink. This light cyan ink has a lightness (L*) of a specific value or more and is employed in place of a normal cyan ink employed in a conventional ink set. Thus, the present invention has been completed.
Accordingly, the present invention provides a water-based ink set for ink-jet recording comprising a cyan ink and a blue ink. The ink set is characterized in that the cyan ink is a light cyan ink having a lightness (L*) of about 60 or more in the L*a*b* calorimetric system.
In another aspect, the present invention provides a water-based ink set for ink-jet recording comprising a cyan ink and a green ink. The ink set is characterized in that the cyan ink is a light cyan ink having a lightness (L*) of about 60 or more in the L*a*b* calorimetric system.
In addition, the present invention provides an ink-jet recording method employing any of the above-mentioned water-based ink sets for ink-jet recording.
The water-based ink set for ink-jet recording of the present invention comprises a light cyan ink having a low coloring agent concentration as a cyan ink. Therefore, when ink-jet recording is performed using this ink set, graininess can be reduced in a low density printed part containing cyan color.
Further, in another aspect, the ink set of the present invention comprises a blue ink in addition to the light cyan ink. Therefore, a color reproduction range in a cyan direction is not reduced in a high density printed part, and the color reproduction range in a blue direction is significantly extended, and the vividness of blue color is improved. The improvement of the vividness of blue color is achieved since blue color is not expressed by color mixing of a cyan ink and a magenta ink but can be expressed by the blue ink alone.
Further, according to the ink set of the present invention, the following effects can be achieved by use of the two inks, i.e., the light cyan ink and the blue ink: the reduction of graininess in a low density printed part containing cyan color; the prevention of the reduction of the color reproduction range of cyan color in a high density printed part; the significant extension of the color reproduction range in the blue direction; and the improvement of the vividness of blue color.
The ink set of the present invention in one aspect comprises a green ink in addition to the abovementioned light cyan ink. In this aspect, the color reproduction range in a cyan direction is not reduced in a high density printed part. In this case, the color reproduction range in a green direction is significantly extended, thereby improving the vividness of the green color. Incidentally, the improvement of the vividness of green color is not expressed by color mixing of a cyan ink and a yellow ink but can be expressed by the green ink alone.
Further, according to the ink set of the present invention, the following effects can be achieved by use of the two inks, i.e., the light cyan ink and the green ink: the reduction of graininess in a low density printed part containing cyan color; the prevention of the reduction of the color reproduction range of cyan color in a high density printed part; the significant extension of the color reproduction range in the green direction; and the improvement of the vividness of green color.
The ink set of the present invention in one aspect comprises a light cyan ink, a blue ink and a green ink. In this aspect, the color reproduction range in a blue direction as well as in a green direction is significantly extended, thereby improving the vividness of the blue color printed by use of the blue ink alone as well as the vividness of the green color printed by use of the green ink alone.
Therefore, according to the ink set of the present invention, the color reproducibility and the print quality of a color image formed by an ink-jet recording method can be improved.
Generally, blue color is the third most frequently used color in text printing next to black color and red color. Green color is also generally used in text printing. According to the present invention, the text in blue or green color is not printed by superposing two inks but can be printed by use of the blue ink alone or green ink alone. Therefore, sharp print quality can be achieved in which feathering is suppressed, and good color developing properties can also be obtained.